Dephosphorization of aluminum alloys by cadmium

ABSTRACT

DEPHOSPHORIZATION OF ALUMINUM AND ALLOYS OF ALUMINUM BY REACTION, WHILE IN A MOLTEN STATE, WITH CADMIUM OR CADMIUM AND LEAD WHEREBY CADMIUM REACTS WITH THE PHOSPHORUS IN THE ALLOY TO FORM CADMIUM PHOSPHORUS WHICH SEPARATES WITH THE CADMIUM FROM THE ALLOY.

United States 3,725,666 Patented Apr. 10, 1973 Free 3,726,666 DEPHOSPHORIZATION OF ALUMINUM ALLOYS BY CADMIUM Pierre Desre, Grenoble, Olivier Berthon, Syessinet, and

Michel Jacquet, Coublevie, France, assignors to Compagnie Pechiney, Paris, France No Drawing. Filed Aug. 18, 1970, Ser. No. 64,803 Int. Cl. C22b 21/06; C22c 21/02 US. Cl. 75-68 R 11 Claims ABSTRACT OF THE DISCLOSURE Dephosphorization of aluminum and alloys of aluminum by reaction, while in a molten state, with cadmium or cadmium and lead whereby cadmium reacts with the phosphorus in the alloy to form cadmium phosphorus which separates with the cadmium from the alloy.

This invention relates to the removal of phosphorus from aluminum and particularly from alloys of aluminum.

Dephosphorization of alloys of aluminum is of considerable industrial importance with respect to aluminum silicon alloys in which silicon is present in the vicinity of eutectic proportions, such as in A 813, A-Sll'ZUN, A- SIOU4, A-SIOG, A-SlO, and A-S7G alloys. When unrefined, such alloys have a granular structure which contains coarse crystals of silicon, an abrasiveness which is harmful to machine tools and it is characterized by mediocre mechanical properties.

Industrially, the machinability charactertistics can be improved by modification treatment with sodium or salts of sodium. In this manner, modification is secured in the micrographic structure and considerable improvement is achieved in mechanical properties. For example, with A- S13, the following results are observed:

by vaporization to leave a practially phosphorus-free aluminum or alloy of aluminum.

Two variations may be used in the practice of this invention. The simplest method is applicable to alloys having melting points substantially below the melting point of cadmium of 767 C. at atmospheric pressure. It consists of melting the alloy and the cadmium in an inert atmosphere and then accelerating the reaction by moderate agitation. The mixture is allowed to rest for a long period of time in order to allow the cadmium and the cadmium phosphorus to settle to the lower portion of the apparatus and then the upper layer of treated alloy is decanted off or else it is separated from the lower portion after the liquid mass has solidified.

Use can be made of cadmium in the elementary state but it is preferred to make use of lead-cadmium alloy which has the effect of reducing the vapor tension of the alloy and phase separation by decantation. It is preferred to make use of an amount of cadmium which greatly exceeds th'e amount required to react with the phosphorus to be eliminated from the alloy. For this purpose, use is made of an amount within the range of 0.5% to by weight of the alloy to be treated. Under these conditions, a rapid and complete reaction is secured to reduce the residual content of phosphorus in the alloy to an amount less than 2 p.p.m. and preferably within the range of 0 to 2 p.p.m.

When reaction has been completed, the cadmium or the lead-cadmium phase, which settles to the bottom, contains most of the phosphorus that has been eliminated from the lighter alloy phase.

In another variant of the invention, dephosphorization is effected by reaction of the cadmium in a vapor phase. This variant is particularly adapted to commercial practice for dephosphorization of aluminum alloy and involves the use of a furnace temperature in the order of 800 C. above the vaporization temperature of cadmium However, the modification is retained by the alloy for only a short time due to the progressive disappearance of sodium in the bath of the liquid alloy. During extended periods of shell casting, as in the manufacture of a large number of parts, renewal by additions of sodium must. be made approximately every half hour.

In commercial alloys, phosphorus is the impurity most responsible for the mediocre properties. It is usually present in an amount within the range of 5 to 20 p.p.m. In the form of aluminum phosphorus, it serves as a seed for the crystallization of silicon and the granular structure. In order permanently to improve the alloy to provide a lamellar structure and good mechanical properties, without renewed refining of the alloy, it is deemed desirable substantially to eliminate the phosphorus from the alloy.

It is an object of this invention to produce an aluminum alloy which is substantially free of phosphorus impurities and to provide a method and means for producing the same.

In accordance with the practice of this invention, dephosphorization of aluminum alloy, and particularly an aluminum silicon alloy, is achieved by reacting cadmium in a molten or vapor state with the molten aluminum al- 10y for transformation of phosphorus in the alloy to cadmium phosphorus, after which the cadmium containing phase is separated from the alloy either by decantation or at atmospheric pressure. It is only by this method that it is practical to treat alloys of aluminum having a high titer of silicon, having a melting point above 767 C.

This method is rendered more efficient when bubbles of cadmium vapor are separated as fine bubbles that are uniformly distributed throughout the molten mass of the alloy to be treated. Introduction of the cadmium vapor can be effected most simply by blowing through a lance of refractory material having a portion immersed in the liquid and which is provided with a large number of fine outlet openings. However, it is difiicult to obtain uniform distribution of the cadmium vapor through the mass of metal alloy.

It is preferred to make use of a furnace or crucible having a bottom wall for the molten metal provided with one or more porous spouts across which the cadmium vapor is passed to issue as very finely divided and well distributed vapor bubbles in the liquid alloy. Such porous spouts, which are impermeable to the molten metal but permeable to the cadmium vapors, are well known from the prior art for introduction of gaseous substances into the liquid media. If the porous spouts are located across the lower portion of the furnace or crucible, the vapors of cadmium can be made available from an outside boiler which is connected to the spouts by insulated passages or pipes.

In accordance with a preferred practice of this invention, use is made of a crucible separated into upper and lower compartments by a horizontally disposed wall in which the porous spouts are fixed. The crucible is filled with the molten alloy in the upper compartment and cadmium is introduced into the lower compartment in a molten state. There it is volatilized to yield vapors of cadmium which pass upwardly through the porous spouts into the bath of molten metal to be dephosphorized in the upper compartment.

The cadmium vapors issuing from the bath of molten alloy are collected and condensed. The process can be carried out at atmospheric pressure but, in order to achieve substantially complete elimination of the last traces of phosphorus, it is preferred to terminate the process with the area above the liquid cadmium alloy under the subatmospheric pressure, as by operating the condenser under suction.

Cadmium phosphorus is somewhat unstable in the gaseous phase whereby the phosphorous continues to go olf as a vapor while the cadmium condenses so that the cadmium phosphorus will not accumulate in the cadmium which is condensed, whereby the latter can be recycled for use in a continuous dephosphorization process.

The invention is applicable not only to aluminum and silicon alloys of aluminum, but also to other aluminum base alloys which may contain phosphorous as an impurity.

The following examples are given by way of illustration, but not by way of limitation, of the practice of this invention:

EXAMPLE 1 In an open enclosure which is protected from air by a flux of argon, a mixture of 1 part by weight of cadmium, 1 part by weight of lead and 2 parts by weight of alpax (an aluminum alloy containing 12% silicon and an initial phosphorus content of 13 p.p.m.), is heated to 650 C.

The molten mixture is agitated mildly for 3 hours to assure good contact of the metals and then the bath is allowed to rest for 16 hours at the same temperature to let the cadrninum, lead and the cadmium phosphorus settle at the bottom to enable separation by decantation.

After cooling, an alpax phase having 2 p.p.m. phosphorus and a lead-cadmium phase containing 12 p.p.m. phosphorous are separated by sawing to separate the upper alloy layer from the lower cadmium layer which can be used again in a new cycle of operation.

The alpax obtained contains approximately 0.06% lead and 0.05% cadmium.

EXAMPLE 2 This example relates to the use of liquid cadmium alone, without the addition of lead.

The operation is carried out in a sealed enclosure provided with a condenser. A mixture of 1 part by weight of cadmium and 40 parts by weight of an alpax alloy having 12% silicon, and containing 14 p.p.m. phosphorus, is heated to a temperature between 700 and 800 C. The mixture is maintained at this temperature for 3 hours with slight agitation.

At the end of the operation, part of the cadmium is recovered on the condenser. The remainder of the cadmium is eliminated from the metal by creating a partial vacuum so as to vaporize all of the cadmium which collects on the condenser. Then the alloy is cast. Its phosphorus content is 2 p.p.m. and its residual cadmium content is about 50 p.m.m.

The condensed cadmium also contains 2 p.p.m. phosphorous since the cadmium phosporous has been decomposed while in the gaseous phase. The mechanical characteristics on an Al-S alloy, prepared from several batches dephosphorized in accordance with Example 2, were measured and the results are set forth in the following table:

Mechanical characteristics Sand cast Shell cast H A, H A, Alloy bar percent bar percent Dophosphorized Al-Si 15. 3 4. 9 18. 1 l0. 4 Dephosphorized Al-Si remelted and kept for 8 hours at. 800 C 14. 9 5. 0 17. 5 10. 8

An examination of this table shows that the dephosphorized alloys no longer have the granular structure, even after prolonged heating at 800 C. They need not be refined with sodium for shell casting.

EXAMPLE 3 In a crucible maintained at 850 C. there is placed 5 kg. of an alloy of aluminum containing 22% silicon and 25 p.p.m. phosphorus. The operation is carried out with a porous plug across the base of the crucible. Pure cadmium vapor is passed at 850 C. through the porous plug in order finely to divide the gas which issues as bubbles into the bath of molten metal which rises through the bath. The cadmium was recovered in a refractory condenser mounted at the upper portion of the enclosure.

The phosphorous content of the remaining alloy is 1 p.p.m. That of the condensed cadmium is less than 2 p.p.m., thereby making it possible to recycle the cadmium. The content of cadmium in the aluminum alloy which is dephosphorized is 200 p.p.m.

4.2 kg. of phosphorous-free aluminum was added to the alloy and after blending the mixture by fusion, 9.2 kg. of a 12% silicon alloy was obtained having properties which are practically the same as those of the alloy of Example 2.

It will be understood that changes may be made in the details of formulation and operation without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. A method for dephosphorization of aluminum alloys which contain silicon and phosphorus, comprising treating the aluminum alloy, while in a molten state, with cadmium in vapor or liquid phase as the sole reagent whereby cadmium reacts with the phosphorus in the aluminum alloy to form cadmium phosphorus compounds, and then separating the molten dephosphorized aluminum alloy from the cadmium phosphorus compounds.

2. The method as claimed in claim 1 in which separation is efiected by allowing the heavy cadmium phase to settle to the bottom while the lighter dephosphorized aluminum alloy component remains at the top.

3. The method as claimed in claim 2 in which the molten aluminum is decanted for separation.

4. The method as claimed in claim 2 in which the melt is allowed to cool to solidify and the upper dephosphorized aluminum portion is separated from the remainder.

5. The method as claimed in claim 1 in which the treatment is carried out at a temperature above the melting point temperature of the aluminum alloy but below the boiling point temperature of cadmium.

6. The method as claimed in claim 5 which includes the addition of lead with the cadmium to reduce the vapor tension of the cadmium and to facilitate separation by decantation.

7. The method as claimed in claim 1 in which the treatment is carried out at a temperature above the boiling point temperature of cadmium and in which the cadmium is introduced into the molten aluminum alloy as a vapor.

8. The method as claimed in claim 7 which includes the step of condensing the vapors of cadmium that rise from the molten metal.

9. The method as claimed in claim 7 in which the cadmium is introduced into the molten alloy through one or more porous spouts positioned in the lower portion of the mass of molten aluminum alloy.

10. The method as claimed in claim 7 which includes the step of terminating the dephosphorization process under subatmospheric pressure to effect substantially complete removal of cadmium from the aluminum alloy by volatilization.

11. The method as claimed in claim 7 in which the process is carried out in an enclosure having a horizontally disposed wall separating the enclosure into an upper compartment and a lower compartment and in which the molten aluminum alloy is in the upper compartment and the cadmium in the lower compartment, and communicating the lower compartment with the upper compartment for transfer of cadmium vapors from the lower compartment to the upper compartment into the molten bath of aluminum alloy.

References Cited UNITED STATES PATENTS 2,034,372 3/1936 Betterton et al 75-68 R 2,073,018 3/1937 Lepp 75-68 R 2,513,339 7/1950 Loevenstein 75-68 R 1,658,702 2/1928 Bernhoeft 75-138 2,031,487 2/ 1936 Kirsebom 75-67 2,383,026 8/1945 Toleik 75-138 2,915,391 12/1959 Criner 75-138 431,912 7/1890 Netto 75-68 R 1,763,248 6/1930 Moore 75-93 E 1,848,797 3/1932 Pacz 75-68 R FOREIGN PATENTS 537,400 6/1941 Great Britain 75-68 HENRY W. TARRING II, Primary Examiner US. Cl. X.R.

75-93 AC, 138, 148, 93 R 

